For various applications utilizing rubber which require high strength and abrasion resistance, particularly applications such as tires and various industrial products, sulfur-cured rubber is utilized which contains substantial amounts of reinforcing fillers. Carbon black is commonly used for such purpose and normally provides or enhances good physical properties for the sulfur cured rubber. Particulate, precipitated silica is also sometimes used for such purpose, particularly when the silica is used in conjunction with a coupling agent. In some cases, a combination of precipitated silica and carbon black is utilized for reinforcing fillers for various rubber products, including treads for tires.
Coupling agents such as, for example, an organosilane polysulfide having an average of from 3.5 to 4 sulfur atoms in its polysulfidic bridge has been used for coupling precipitated silica to elastomers.
Exemplary of such organosilane polysulfide is bis-3(triethoxysilylpropyl) polysulfide with an average of about 3.8 sulfur atoms in its polysulfidic bridge. It is envisioned that such polysulfide can be a sulfur donor, by liberating free sulfur, during typical high shear mixing of a rubber composition at an elevated temperature such as, for example, at temperatures of 100.degree. C. and above, depending somewhat upon the polysulfide used and the mixing temperature and time.
The small amount of free, liberated, sulfur is then available to combine with and/or possibly partially vulcanize, a diene-based elastomer.
It is, however, considered herein that an organosilane polysulfide compound, which is primarily a disulfide, having an average of about 2.6 or less sulfur atoms in its polysulfidic bridge, is not normally a good sulfur donor under such mixing conditions, due to the relatively strong sulfur-to-sulfur bonds typical of an organosilane disulfide--as compared to an organosilane polysulfide with an average of at least 3.5 sulfur atoms in its polysulfidic bridge.
Accordingly, it is considered herein that, for an organosilane polysulfide compound (disulfide) which contains an average of less than 2.8 and particularly within a range of about 2 to about 2.6, sulfur atoms in its polysulfidic bridge, the liberation of free sulfur, if any, occurs at a relatively slow rate during a high shear rubber mixing stage, even at a mixing temperature in a range of about 150.degree. C. to about 185.degree. C. depending somewhat upon the overall mixing conditions, including the mixing time itself.
Bis-(3-triethoxysilylpropyl) disulfide, as a variety of organosilane disulfide, is also taught as being useful in a silica-containing sulfur vulcanizable elastomer composition, even as a high purity form of such disulfide in, for example, U.S. Pat. No. 4,046,550 and German Patent Publication DT 2,360,471. However, it is considered herein that such disulfide does not ordinarily readily liberate free sulfur in such aforementioned rubber/silica/coupler mixing operation.
For examples of organosilane polysulfides for use as silica couplers, see U.S. Pat. Nos. 4,076,550; 4,704,414; and 3,873,489.
For examples of organosilane disulfides added in a preparatory, non-productive, rubber composition mixing stage, along with a small amount of free sulfur, see U.S. Pat. Nos. 4,076,550; 5,580,919 and 5,674,932.
In practice, sulfur vulcanized elastomer products are typically prepared by thermomechanically mixing rubber and various ingredients in a sequentially step-wise manner followed by shaping and curing the compounded rubber to form a vulcanized product.
First, for the aforesaid mixing of the rubber and various ingredients, typically exclusive of free sulfur and sulfur vulcanization accelerators, the elastomer(s) and various rubber compounding ingredients are typically blended in at least one, and usually at least two, sequential, preparatory thermomechanical mixing stage(s) in suitable mixers, usually internal rubber mixers. Such preparatory mixing is often referred to as "non-productive mixing", or "non-productive mixing steps or stages". Such preparatory mixing is usually conducted at temperatures in a range of about 140.degree. C. to 190.degree. C. and more often in a range of about 140.degree. C. or 150.degree. C. to about 185.degree. C.
Subsequent to such sequential, preparatory mix stage(s), free sulfur and sulfur vulcanization accelerators, and possibly one or more additional ingredients, are mixed with the rubber compound, or composition, in a final, productive, mix stage, typically at a temperature within a range of about 100.degree. C. to about 130.degree. C., which is a lower temperature than the temperatures utilized in the aforesaid preparatory mix stage(s) in order to prevent or retard premature curing of the sulfur curable rubber, which is sometimes referred to as "scorching", of the rubber composition.
Such sequential, non-productive, mixing steps, and the subsequent productive mixing step are well known to those in the rubber mixing art.
By thermomechanical mixing, it is meant that the rubber compound, or composition of rubber and rubber compounding ingredients, is mixed in a rubber mixture under high shear conditions where it autogeneously heats up, with an accompanying temperature rise, as a result of the mixing primarily due to shear and associated friction within the rubber mixture in the rubber mixer.
Such thermomechanical rubber compound mixing procedure and associated sheer and accompanying temperature rise aspects are well known to those having experience in the rubber preparation and mixing art.
In practice, it is believed that the inventors' prescribed procedure of (1) adding an organosilane disulfide in a non-productive rubber composition mixing stage followed by (2) subsequently adding an organosilane polysulfide with an average of from 3.5 to 4.5 sulfur atoms in its polysulfidic bridge together with a small amount of free sulfur in a productive rubber composition mixing stage for a silica-based reinforced rubber composition, particularly as a means of controlling the associated sulfur/elastomer interaction as well as interaction with a silane/silica network, or product, created by reaction of the disulfide in the prior, preparatory, mixing stage(s) is novel and inventive in view of past practice.
In one aspect, it is believed that a decoupling of an initial silane/silica reaction with a subsequent release of free sulfur, and an additional silane reaction, to interact with the elastomer(s) and silane/silica network in a sequential rubber composition mixing procedure is accomplished by using a combination of separate and selective addition of an organosilane disulfide preliminary mixing stage and addition of an organosilane polysulfide followed by vulcanizing the rubber composition according to the procedure of this invention is a significant departure from past practice.
The term "phr" as used herein, and according to conventional practice, refers to "parts of a respective material per 100 parts by weight of rubber, or elastomer".
In the description of this invention, the terms "rubber" and "elastomer" if used herein, may be used interchangeably, unless otherwise prescribed. The terms such as "rubber composition", "compounded rubber" and "rubber compound", if used herein, are used interchangeably to refer to "rubber which has been blended or mixed with various ingredients and materials" and "rubber compounding" or "compounding" may be used to refer to the "mixing of such materials". Such terms are well known to those having skill in the rubber mixing or rubber compounding art.
A reference to an elastomer's "Tg", if used herein, refers to a "glass transition temperature" which can be determined by a differential scanning calorimeter at a heating rate of 10.degree. C. per minute.